Sync pods asynchronously in the Kubelet.

This makes two main changes:
- Runs syncPod in a separate Go routine (and enforces only one of those
  runs at a time).
- Uses the pod list to determine if a container should be running or
  should be killed (used to use the output of syncPod).

Since Docker pulls are synchronized by the Docker daemon we still block
on that, but pods can now be removed and prepared for starting without
blocking on long pulls.
It would just lengthen the critical section which shouldn't be an issue (I guess it's a bit odd since it spawns a go routing that does take the lock, but that *shouldn't* be a problem, tested manually and seems fine). Made the change, PTAL.